In general, an image sensor is a semiconductor device for converting an optical image into an electric signal, and is mainly classified as a Charge Coupled Device (CCD) image sensor or a Complementary Metal Oxide Semiconductor (CMOS) image sensor.
The CMOS image sensor includes a photodiode for detecting light and a CMOS logic circuit for converting detected light into electric signals to form image data. As the quantity of light received in the photodiode increases, the photosensitivity of the image sensor is improved.
To improve the photosensitivity, either a fill factor, which is a ratio of a photodiode area to the whole area of the image sensor, must be increased, or a photo-gathering technology is used to change the path of light incident onto an unintended area such that the light can be gathered in the photodiode.
A representative example of the photo-gathering technology is to make a microlens. That is, a convex microlens is formed on a top surface of the photodiode using a material having superior light transmittance, thereby refracting the incident light in such a manner that a greater amount of light can be transmitted into the photodiode area.
In this case, light parallel with an optical axis of the microlens is refracted by the microlens so that the light is focused on a certain position of the optical axis.
Hereinafter, a conventional CMOS image sensor is described with reference to the accompanying drawing.
FIGS. 1a to 1d are cross-sectional views illustrating the procedure of manufacturing a conventional CMOS image sensor.
Referring to FIG. 1a, an interlayer dielectric layer 13 is formed on a semiconductor substrate having a plurality of light sensing elements, e.g., photodiodes 12.
Here, the interlayer dielectric layer 13 may be formed as multiple layers. Although not shown in the figure, after one interlayer dielectric layer is formed, a light shielding layer is formed to prevent light from being incident onto a portion other than the photodiode region, and then another interlayer dielectric layer is formed thereupon.
Then, a dyeable resist is applied on the interlayer dielectric layer 13, and color filter layers 14 for filtering light for each wavelength band are then formed by performing an exposure and development process.
Subsequently, a planarization layer 15 is formed on the color filter layers in order to adjust a focus distance and secure a planarity for forming a microlens layer.
As shown in FIG. 1b, a resist layer 16a for microlenses is applied on the planarization layer 15, and a reticle 17 with openings is aligned above the resist layer 16a. 
Subsequently, the resist layer 16a is selectively exposed to correspond to the openings of the reticle 17 by radiating light such as laser onto the semiconductor substrate 11 through the reticle 17.
Referring to FIG. 1c, the exposed resist layer 16a is developed to form microlens patterns 16b. 
Referring to FIG. 1d, convex shaped microlenses 16 are formed by reflowing the microlens patterns 16b at a temperature of 150 to 200° C.
The micro lenses 16 allow a larger amount of light to be focused onto the photodiodes 12 by passing the light through the color filter layers 14 according to the wavelengths thereof, when a natural light is incident into the micro lenses 16.
The light incident into the image sensor is focused by the micro lenses 16 and filtered through the color filter layers 14, and then the filtered light is incident onto each photodiode 12 which is correspondingly located below each color filter layer 14.
In a case where such a conventional CMOS image sensor is used for a Digital Still Camera (DSC) in which a thin color pixel is required, a process of forming a color filter layer with a high thickness is performed in order to implement color reproduction.
Accordingly, since all color filter layers for DSC CMOS image sensors have been currently formed in a range of 0.7 to 1.0 μm, color reproduction of colors is low, and sensitivity is the chief consideration. If a color pixel with a high thickness is formed by a one-time process when performing a process of forming a thin color filter layer described above, in particular a blue color filter layer, transmittance for an in-line exposure wavelength (when coating a color photoresist in 0.9 μm or more) is 0.5% or less, and reactivity for light is almost close to zero at a portion contacted with the interlayer dielectric layer 13. For this reason, there is a problem in that the peeling of a color pixel occurs.